This invention relates to a roller furnace for firing ceramic or refractory products, such as tiles and the like, which is provided with a passage running parallel to, and arranged below the firing furnace, in which products are heat treated possibly by the utilization of hot gases removed from the firing furnace.
Processes for producing ceramic articles may be said to fall into two broad categories of types : the single-firing type and the double-firing type.
In a single-firing or -baking process, the pressed, dried and possibly glazed product is baked only once.
In a double-firing process, on the other hand, the pressed and dried product is subjected to a first firing operation to obtain the so-called biscuit, and the biscuit is glazed and, after drying of the glaze, subjected to a second firing operation, or refiring.
By the double-firing process, products are obtained which are improved qualitatively, such for example as facing tiles or even paving tiles or slabs that are decorated on their visible face.
Both the single-firing and double-firing processes are widely used by the ceramic industry in order to answer different market-demands.
The drying as well as the `bi-baking` of tiles are operations being usually carried out on equipments separated from the firing furnace proper.
In order to increase versatility of the roller furnaces, U.S. Pat. No. 4,285,669 has proposed to provide a drying passage below the firing furnace and to supply it with hot gases obtained from several sections of the furnace.
Such an installation is used in particular for single-firing operations and has an advantage in that at the same time as tiles travelling in the furnace are fired, other tiles moving in the underlying dryer are dried.
The refiring operation that is necessary for producing bi-baked tiles, is at present carried out on a separate installation that may be a tunnel furnace or kiln having support plates for the tiles, or even a roller furnace.
Such a type of installation cannot be used for productions other than that involving the forming of a biscuit.
Thus, with to-day's installations, there is no possibility of diversifying the production that is, effecting drying of the tiles by single firing and bi-baking the tiles by refiring them.